When an undercarriage is in the deployed position, prior to being raised, it presents a maximum length, given that the shock absorber fitted to the undercarriage is fully relaxed. Unfortunately, this length may be longer than the length available in the undercarriage bay. It is then appropriate to fit the undercarriage with a shortener device that enables it to be inserted into the bay at the end of being raised.
Various devices are known. For example, the main undercarriage of the Airbus A340 is provided with a shock absorber that is movable inside the strut of the undercarriage between an extended position when the undercarriage is deployed, and a retracted position at the end of raising the undercarriage, thus having the effect of shortening the total length of the undercarriage, without compressing the shock absorber.
That device is complex and heavy since it requires a shock absorber to be provided that can slide inside the strut. Furthermore, the linkage that controls the movement of the shock absorber is subjected to landing forces in full, which means that it needs to be dimensioned accordingly.
In other shortener devices, the shock absorber is compressed in order to reduce the total length of the undercarriage. For example, undercarriages are known that include a telescopic link having its bottom end secured to the slide rod of the shock absorber and its top end secured to the strut, more precisely to an attachment point that is movable on the strut between a landing position and a raising position. When the undercarriage is deployed, the attachment is in a landing position in which the telescopic link is fully relaxed so that the shock absorber is itself fully relaxed. On landing, the telescopic link is compressed together with the compression of the shock absorber. While the landing gear is being raised, the attachment of the telescopic link is brought progressively to a raising position, thereby exerting a traction force on the telescopic link which tends to compress the shock absorber and thus shorten the undercarriage. The movement of the attachment of the telescopic link from the landing position to the raising position may be obtained in various ways. It is possible to make use of the movement of the side brace arm while raising the undercarriage, or it is possible to mount the attachment of the telescopic link to a lever that is hinged to the strut, which lever is itself connected to a fixed point of the structure of the aircraft by means of a link such that raising the undercarriage causes the lever to pivot, thereby causing the attachment point of the telescopic link to move from the landing position to the raising position.
In other shortener devices, the telescopic link is replaced by a simple slide rod in a bearing that is mounted to move on the strut between a landing position and a raised position. The bearing is moved by making use of the relative movement of the strut while the undercarriage is being raised.
Those various devices share the feature of being quite difficult to design since it is necessary to provide specific hinge points on the strut. The linkage can be complex to define.